RUS works hand in hand with GAL (Global Address List).  
Together they generate the list of addresses that users see 
in Outlook.  I think of Exchange 2003's RUS as a little 
engine which runs an LDAP query, the results are to build 
or update the Users' property sheets and the Address Lists.

Here is a list of the jobs that RUS performs:

Updates proxyAddresses attribute controlled by recipient 
policies. 
Initializes the homeMDB, homeMTA and msExchHomeServerName 
attributes.  Also the legacyExchangeDN and 
msExchMailboxGUID if appropriate.
Sets the showInAddressBook (or hideDLMembership).
Sets the ACL on the Microsoft Exchange System Objects 
(Check with ADSI Edit)
Populates the group called Exchange Enterprise Servers in 
Active Directory.

RUS (Recipient Update Service) is responsible for making 
updates to e-mail addresses, and it does this based on 
recipient policy changes. These updates are made at a 
specific interval that is defined for the service. You can 
view the update interval and modify it as necessary by 
completing the following steps:


Start System Manager, and then in the left pane (the 
console tree), click the plus sign (+) next to the 
Replication node. Then select Recipient Update Service. 

You should now see the available Recipient Update Service 
in the right pane. You will have an enterprise 
configuration service and one or more additional services 
for additional domains in the domain forest. 

Right-click the service you want to work with, select 
Properties, and then use the properties dialog box to view 
the service's configuration settings. 

Use Update interval to choose a new update interval. The 
following options are available: 
Always Run 
Run Every Hour 
Run Every 2 Hours 
Run Every 4 Hours 
Never Run 
Use Custom Schedule

rural utilities services.RUS (Recipient Update Service) is 
responsible for making updates to e-mail addresses, and it 
does this based on recipient policy changes